Acquired immune deficiency syndrome (AIDS) is a serious disease of major public health importance. Thus far, an etiologic agent has not been found. Immune electron microscopy, which is the direct observation of antigen-antibody interaction, has been employed as a means for detecting the fastidious etiologic agents of several diseases such as the 27 nm Norwalk agent of acute nonbacterial gastroenteritis, and the 27nm hepatitis A virus of hepatitis. Selected specimens from AIDS patients will be examined by immune electron microscopy or conventional electron microscopy in an attempt to detect the etiologic agent. Presently, collaborative studies using electron microscopy are being carried out on simian AIDS in collaboration with the NINCDS.